Nuclear reactors cooled by pressurized water include steam generators that heat and vaporize feed water by heat exchange with the pressurized cooling water of the nuclear reactor. The cooling water circulates through a primary portion of the steam generator comprising the inside volume of heat-exchange tubes in a bundle of tubes and one or more water chambers serving to distribute the primary cooling water amongst the tubes in the bundle and to recover the primary cooling water that has flowed along the tubes. The steam generator also comprises an outer casing surrounding the bundle and defining a secondary portion of the steam generator into which feed water is introduced and caused to flow in contact with the outside surfaces of the tubes in the bundle in order to be heated and vaporized, the resulting steam being recovered in the secondary portion of the steam generator for delivery to the power station turbine, after separating out residual water and drying.
Steam generators of pressurized water nuclear reactors may have a bundle of U-shaped tubes whose ends are fixed in holes passing through a tube plate so that each tube has a first end opening out into a first compartment of the water chamber fixed beneath the tube plate, and a second end opening out into a second compartment of the water chamber. The two compartments of the water chamber are separated from each other by a partition plate. Under such circumstances, the water chamber constitutes the bottom portion of the steam generator when it is in its in-service position, with the axis of the steam generator being vertical, which axis is common to the outer casing of generally cylindrical shape, to the bundle of tubes, and to the tube plates.
The steam generators of pressurized water nuclear reactors may also be generators having straight tubes, with the ends of each tube in the bundle of parallel tubes being fixed in respective holes passing through first and second tube plates, with first and second water chambers being fixed respectively to the first and second tube plates. The steam generator further comprises an outer casing surrounding the bundle and defining the secondary portion of the steam generator between the first and second tube plates.
In its in-service position, the axis of the steam generator is vertical. One of the water chambers is disposed at the bottom end of the steam generator, beneath the first tube plate, and the second water chamber is placed above the second tube plate, at the top end of the steam generator. The first water chamber shares the primary cooling water of the nuclear reactor amongst the tubes of the steam generator bundle, and the second water chamber fixed to the second tube plate at the top end of the steam generator recovers the cooling water that has flowed along the tubes in the bundle.
The water chambers of steam generators in pressurized water nuclear reactors have a circularly symmetrical wall that is generally hemispherical or cylindrical-and-hemispherical, which wall is assembled to the corresponding tube plate in a coaxial disposition.
The circularly symmetrical wall of each water chamber is pierced by an opening to allow primary cooling water to enter into the water chamber or to allow cooling water to be evacuated out from the water chamber, and also by openings constituting inspection holes or manholes in the water chambers.
The water chambers of steam generators located at the bottom ends of the generators, i.e. the single two-compartment water chamber for a steam generator having U-shaped tubes, or the water chamber in a generator having straight tubes that serves to distribute the primary cooling water amongst the tubes of the bundle, have walls in which the through openings are offset laterally away from the axis of the steam generator, with the axes of these openings being inclined relative to the axis of the steam generator.
For a steam generator having U-shaped tubes, each of the compartments of the water chamber has one opening therein receiving a coupling tube for coupling to the primary circuit of the nuclear reactor, and another opening constituting a manhole.
For steam generators having straight tubes, the water chamber situated at the bottom end of the steam generator has one or two openings therein receiving one or two tubes for coupling to the primary circuit, and at least one inspection opening.
The wall of a water chamber disposed at the bottom end of a steam generator has a central portion around the axis of the steam generator between the inside edges of the through openings situated beside the axis of the steam generator, which central portion is entirely solid and is referred to below as the “bottom” of the steam generator. The inside surface of the bottom of the steam generator is concave and can be defined by a linear outline that is tangential to the insides of the through openings in the wall of the water chamber. For a steam generator having U-shaped tubes and a hemispherical water chamber, the partition plate is substantially semicircular in shape, lying on a plane containing the axis of the steam generator and resting on the bottom of the steam generator on a diametral zone of the inside surface of the bottom.
When a maintenance or repair operation is performed on a steam generator after the nuclear reactor has been stopped, it is necessary to empty the steam generator completely, after cooling down and depressurizing the cooling water contained in the primary circuit.
Because of the presence of the concave inside surface of the bottom of the lowest portion of the steam generator, some primary cooling water of the nuclear reactor remains in this bottom portion of the steam generator and cannot be removed via the connection tubes or the manholes. As a general rule, channels referred to as “drains”, are provided which pass through the wall of the water chamber between the inside portions of the connection tubes or of the manholes and the concave inside surface of the bottom of the steam generator. However, it is not possible to machine drains in the wall of the water chamber or in portions fitted to the wall in such a manner as to be able to suck out all of the primary cooling water contained in the bottom of the steam generator. The residual cooling water contains activated particles, such that the operations of maintaining and repairing a steam generator are made more complex and the dosage rate to which the operators performing maintenance or repair are subjected can be quite high.
It is therefore desirable to avoid having any residual cooling water present after emptying out steam generators, or more generally heat exchangers likely to contain substances that emit radiation or substances that are toxic.